This invention relates to processes for preparing phosphonic acids and, more particularly, to an improved process for preparing 1-hydroxy, ethylidene-1,1 -diphosphonic acid.
Various processes have been suggested in the past for the production of such 1-hydroxy, organo-1,1-diphosphonic acids as 1-hydroxy, ethylidene-1,1-diphosphonic acid but all of these prior suggested processes suffer from one or more disadvantages, either technical or economic. Most of the prior art processes for the production of these diphosphonic acids are carried out batch-wise and do not lend themselves to continuous operation. They likewise suffer from the disadvantage of gross underutilization of reaction equipment in most instances and relatively long and inefficient process cycle times. Such processes have involved handling and storage of large amounts of carboxylic acid halides which in such concentrations pose safety hazards in the handling thereof.
Even those prior art processes which are adaptable to continuous operation suffer certain disadvantages which render them unattractive for commercial installation. For example, the process disclosed in U.S. Pat. No. 3,366,677 of Quimby utilizes the expensive reagents phosphorous acid and acetic anhydride as well as acetyl chloride and requires the use of from 3 to 8 times the molar ratio of acetic anhydride to phosphorous acid making the process particularly expensive to carry out. Further prior art continuous processes such as that disclosed in U.S. Pat. No. 3,449,409 of Irani and Mitchell require operation at a temperature sufficiently low as to contain a two-phase reaction mixture containing a separate liquid phase of carboxylic acid halide and, subsequently thereto, the removal, storage and recharging of such acid halide to the reaction mixture in a subsequent reaction vessel. These procedures lead to disadvantages. The handling, storage and recharging of the acid halide constitutes a safety hazard as mentioned hereinabove. Thus, improved processes which can be carried out continuously with improved equipment utilization, improved safety and shorter reaction cycle times have been desired.